A single-switch three-phase discontinuous conduction mode ("DCM") boost converter is widely used in three-phase power factor correction applications, mainly due to its uncomplicated design and simple control circuit. There are, however, certain drawbacks in its use, primarily because the circuit has a high content of harmonic currents with a corresponding harmonic distortion unless the input peak voltage is significantly lower than the output voltage. In the case of a 800V output converter with a 380V rms input, 800V output converter, the input power must be limited to amounts below 6 kW in order to meet IEC 1000-3-2 harmonic current requirement with the traditional pulse width modulated ("PWM") control.
In order to expand the power capability of a single-switch three-phase DCM boost converter, several alternative approaches have been suggested. One proposal made was to control the converter in a critical condition mode, that is, when the current in the freewheeling diode reaches zero, the main switch is turned on again. This concept is covered in J. W. Kolar, H. Ertl, and F. C. Zach, "Space Vector-Based Analytical Analysis of the Input Current Distortion of a Three-Phase Discontinuous-Mode Rectifier System," Records of Power Electronics Specialist Conference, pp. 696-703, 1993, a copy of which is incorporated herein by this reference. The very wide, load-dependent switching frequency of this proposal, however, made the design of the electromagnetic interference ("EMI") filter very difficult.
In a DCM converter, the high-frequency averaged current is proportional to the square of the duty cycle of the main switch divided by the switching frequency (d.sup.2 /f.sub.s). This means the input current waveform can be improved by either changing the duty cycle or switching frequency. The possibility of modulating the main switch duty cycle with a harmonic waveform has been investigated. Q. Huang and F. C. Lee, "Harmonic Reduction in Single-Switch, Three-Phase Boost Rectifier With High Order Harmonic Injected PWM," in VPEC Seminar Rec., Blacksburg, Va., September 1995, a copy of which is incorporated herein by this reference. In this case a multiplier is required because the modulated waveform should be proportional to the dc component of the duty cycle, which changes with the load power.
Accordingly, what is needed in the art is an expansion of the power capability of a single-switch three-phase DCM boost converter, without resorting to complex circuitry and control schemes, where EMI filtering requirements can be reduced and a multiplier is not required for a modulated waveform.